1. Field of the Invention
The present invention relates to a photo process system for use in the manufacturing of semiconductor wafers. More particularly, the present invention relates to a photo process system for forming a high quality photoresist film on a semiconductor wafer.
2. Description of the Related Art
The fabrication of semiconductor devices includes a photo process comprising steps of forming a photoresist film on a wafer, exposing the photoresist film to define a predetermined pattern thereon, and developing the exposed photoresist to form a film of the predetermined pattern on the wafer.
In clustered systems whose use has been increasing as of recent, all of the units of equipment for performing the photo process, for example, a coating unit, a developing unit, a baking unit, and the like, are clustered in one place. Therefore, the distance between the units, and hence, the time required to move a wafer from one unit to the next, is relatively short. Thus, the photo process can be performed efficiently by such equipment.
FIG. 1 is a schematic of the layout of a conventional clustered photo process system.
As shown in FIG. 1, a robot 110 for transferring a wafer is installed at the center of a housing 100. The robot 110 is surrounded by a coating unit 120, a developing unit 130, a baking unit 140, a loading/unloading unit 150, and a controller 160.
The coating unit 120 executes a spin coating method in which the wafer is rotated at a predetermined speed, and the rotating wafer is coated with a photoresist. The photoresist is dispersed uniformly over the surface of the wafer due to the centrifugal force of the rotating the wafer.
The developing unit 130 develops a pattern formed on the photoresist film in the exposure step, and the baking unit 140 bakes the wafer at a predetermined temperature between the above-described steps.
The loading/unloading unit 150 both loads the wafer into the system and unloads a completely processed wafer from the system. The loading/unloading unit 150 is designed to support several wafer carriers at a time.
The controller 160 includes a computer system for controlling the units of photo processing system. The robot 110 has a support 111 fixed to the bottom surface of the housing 100, and an arm 112 which is integrated with the support 111 and can move freely. The arm 112 moves a wafer between the units within the housing 100.
The steps of the photo process performed by the conventional photo process system will now be described generally.
In a pre-baking step (1), a wafer is introduced via a wafer carrier into the loading/unloading unit 150. From there, the wafer is transferred to the baking unit 140 by the robot 110. In the baking unit 140, the wafer is heated to a predetermined temperature which causes organic material or foreign material to evaporate from the surface of the wafer.
In a photoresist (PR) coating step (2), the resultant wafer is transferred to the coating unit 120. A photoresist film is formed on the surface of the wafer by the coating unit 120.
In a soft baking step (3), the wafer is transferred back to the baking unit 140 where the wafer is heated for a predetermined amount of time. In this step, the photoresist is dried so that it attaches firmly to the surface of the wafer.
In an exposure step (4), the wafer is transferred to an exposure unit, such as a stepper or scanner. The photoresist film is photo-sensitized by the exposure unit to define a pattern thereon.
In a post-exposure baking (PEB) step (5), the exposed wafer is transferred back to the baking unit 140. The wafer is again heated for a predetermined amount of time by the baking unit 140.
In a developing step (6), the wafer is cooled to a temperature within a predetermined range. Subsequently, the photoresist pattern is developed by the developing unit 130.
In a hard baking step (7), the wafer is heated so that the photoresist pattern will be even more firmly attached to the surface of the wafer.
The above-described steps are performed in a clean environment in which the temperature and humidity are controlled, and in which dust or other foreign materials have been eliminated. Thus, the photo process system is installed within a clean room provided with an air conditioner.
In the PEB step, optically-decomposed resins are rearranged due to thermal diffusion by heating the exposed photoresist at a predetermined temperature, thus cleaning the profile boundary (cross section) between exposed patterns. This PEB step is performed to prevent abnormalities from being produced in the profile of the patterns and non-uniformity of the critical dimension of the patterns when the patterns are irradiated with ultraviolet (UV) or deep ultraviolet (DUV) light during the exposure process. Because light irradiating the exposed portion diffracts and produces interference according to the reflectivity and refractivity of the wafer and the optical absorption level of the photoresist, the diffraction and interference of the light would create the above-mentioned abnormalities and non-uniformity in the exposed portion if the resins were not rearranged by being heated prior to exposure.
The problems posed by these optical phenomena of diffraction and interference can alternatively be solved by coating the photoresist film with an anti-reflection film prior to exposure. However, the PEB process is used more often.
When the exposure light in the photo process is a DUV light, a chemically-amplified resist is used as the photoresist. A portion of the chemically-amplified resist, which is exposed by thermal treatment, changes into an acid which is soluble in a developing solution. Also, the alteration of the chemically-amplified resist occurs due to a chain reaction, so that the balance of heat applied to the entire wafer in the PEB step has the greatest effect on the uniformity of the critical dimension of a pattern.
FIG. 2 is a schematic diagram of a conventional photo process system installed in a clean room.
Referring to FIG. 2, the clean room typically has a passageway 10 and an equipment installation zone 20. The photo process system is located in the equipment installation zone 20, adjacent to the passageway 10. The upper portion of the clean room is provided with an upper plenum 30 through which clean air enters the room, and the lower portion of the clean room is provided with a lower plenum 40 through which air is discharged from the room. Filters 31 are installed between the upper plenum 30 and the equipment installation zone 20 and passageway 10 of the clean room. The photo process system has a housing 100, a coating unit 120 disposed in the housing 100 at one side thereof, and baking unit shelves 140a disposed in the housing 100 at the other side thereof. A plurality of baking units 140 are installed in multiple stages on the baking unit shelves 140a. A robot 110 is installed at the center of the housing 100.
The air conditioner of the photo process system includes a blower 171, blast pipes 172, and an air filter 173. The blower 171 is installed within the lower plenum 40, the air filter 173 is installed in the upper portion of the housing 100, and the blast pipes 172 extend from the blower 171 to the upper portion of the housing 100 in which the air filter 173 is provided. The blower 171 typically includes a chemical filter for removing NH3 from the air.
In the clean room, dust in the equipment installation zone 20 is a prevented from entering the housing 100, and different pressures are maintained in different areas of the clean room to prevent fumes or the like from diffusing from one area to another. For example, the pressure in the passageway 10 is about 0.17 to 0.18 mmH2O greater than that in the upper area of the housing 100. The pressure in the passageway 10 is about 0.07 to 0.08 mmH2O greater than that in the housing 100. The pressure in the housing 100 is about 0.09 to 0.12 mmH2O greater than that in the equipment installation zone 20. The pressure in the equipment installation zone 20 is about 0.60 to 0.70 mmH2O greater than that in the lower plenum 40. The pressure in the passageway 10 is greater by about 0.71 to 0.82 mmH2O than that in the lower plenum 40. The pressure in the housing 100 is greater by about 0.02 to 0.082 mmH2O than that in the baking unit 140. The pressure in the baking unit 140 is greater by about 0.06 to 0.08 mmH2O than the pressure at the exterior of the rear surface the baking unit 140 .
Due to these pressure differences, clean air flows from the upper plenum 30 to the lower plenum 40 via the passageway 10, the interior of the housing 100, and the equipment installation zone 20. The bottom of the passageway 10 and the housing 100 is typically formed of gratings, and the clean air flows through these gratings into the lower plenum 40. Clean air entering the housing 100 from the passageway 10 passes sequentially through the coating unit 120 and the front surface, the interior and the rear surface of the baking unit 140, and enters the lower plenum 40 through the rear surface of the housing 100.
FIG. 3 shows the flow of clean air within a conventional photo process system.
As shown in FIG. 3, clean air flowing through the blast pipes 172 enters the housing 100 via the air filter 173. Here, the amount of clean air entering the housing 100 is controlled by dampers 174 provided in the blast pipes 172. The clean air entering the housing 100 forms a downwardly flowing air current within the housing 100. A grating 101 at the lower portion of the housing 100 allows clean air to pass therethrough, but the bottom surface 102 of the housing 100 is completely closed. Consequently, the clean air is discharged to the outside of the housing 100 via air vents 103 provided on the lower rear surface of the housing 100. Hence, clean air enters a baking unit shelf 140a on which a baking unit 140 is installed, from the front portion of the baking unit, and is discharged out the rear portion of the baking unit.
In the prior art, the upper surface of a highest baking unit shelf 140a is spaced a predetermined distance below the air filter 173, whereby an empty space 140b is formed between the highest baking unit shelf 140a and the air filter 173. Accordingly, clean air entering the space 140b impinges the upper surface of the highest baking unit shelf 140a and becomes turbulent. Then, the turbulent clean air exits the space 140b and joins the downwardly flowing air current. The turbulent clean air at the front portion of the baking unit shelf 140a is so excessive that it impedes the clean air from flowing downwardly as a laminar flow of air. The turbulent air current induced adjacent to the baking unit shelf 140a flows into the baking unit 140 having a lower pressure.
Therefore, the flow of clean air is not completely laminar in the housing 100 of the conventional photo process system. Also, the way in which clean air flows into the baking unit shelves 140a on which the baking units 140 causes parts of the baking unit shelves 140a to have different temperatures. Thus, parts of a wafer loaded within the baking unit 140 are baked at different temperatures, respectively. For example, as air of a temperature lower than that in the baking unit flows into the baking unit from the front portion thereof, a part of the wafer closest to the front portion of the baking unit shelf 140a comes into contact with the clean air, whereby the temperature of the part of the wafer closest to the front portion of the baking unit shelf 140a becomes less than that of the other part of the wafer.
Furthermore, the baking unit 140 is clustered with a cooling water line and a variety of utility pipes and boards which cause the temperature within different parts of the baking unit 140 to even further deviate from the norm. According to experiments, a cooling water line adjacent the baking unit 140 causes the baking unit shelf 140a to have internal temperatures differing by about 1xc2x0 C. This non-uniformity in the temperature results in non-uniformity in the thickness of the photoresist film and in the critical dimension in the photoresist pattern.
An object of the present invention is to provide a photo process system for semiconductor wafers which can produce a photoresist film having a highly uniform thickness and a pattern from the photoresist having a highly uniform critical dimension.
Accordingly, to achieve the above object, the present invention provides a photo process system for semiconductor wafers in which clean air entering the system from an upper portion of a housing, in which the baking units are located, is constrained to flow downward as a laminar air current within the housing past the front of the baking units. To produce the laminar air flow, the discharge opening is formed at the bottom of the housing, and an air flow containment dam prevents clean air entering said housing from flowing horizontally at the top of the baking region. Such a horizontal flow of air would otherwise produce turbulence severely affecting the ability of the air to flow downward across the front of the baking units and toward the discharge opening as a laminar air flow.